The actor appeared at the Department of Defense headquarters to chat about his newest film, care of the kind of press event usually reserved for actual government functions.

In one of the more off-beat movie marketing choices of late, “Hunter Killer” star Gerard Butler appeared at the Pentagon — the Virginia headquarters of the Department of Defense, usually the part of the federal government most closely aligned with national security and the armed forces, not film-centric interview opportunities — earlier this week to chat about his upcoming submarine thriller. Butler was on hand to answer queries about his film, which follows a group of Navy SEALs who are forced to save the kidnapped Russian president in an effort to avert a worldwide military crisis, thanks to the full support and involvement of both the U.S. Navy and Department of Defense.

The actor shared that he spent three days in a Navy submarine at Pearl Harbor, an experience he likened to “entering another country, another world — it’s like an alien planet.” As part of his preparation, Butler dove into every aspect of sub life, and he added that he “was shipped from sonar to the bridge, to navigation to the engine room to the torpedo room because I had a very quick-minded sub commander who wanted to show me every working, living part of the submarine — even how to compress trash.”

The film’s official production notes give a deeper explanation of why exactly the Navy and DoD assisted with the creation of a piece of popcorn entertainment, noting that “the filmmakers secured an early agreement to partner with the U.S. Navy in nearly every aspect of the production.” The Donovan Marsh-directed film had Navy technical advisors on set “at all times,” all the better to get every detail correct. U.S. Navy consultant Russell Coons even took the cast and crew into a damage control trainer, built to simulate different battle emergencies that a sub’s crew might face, from flooding to fire.

The Navy let the production spend two days actually filming on a working nuclear submarine docked at Pearl Harbor, with one day dedicated to interior shots and another spent at sea. And while the film only includes those two days of actual on-sub footage, the production created an entire submarine set from Navy-approved photographs from the real sub, aided by 3D printing technology.

Butler had some glowing words for the military men who made the film possible. He told the crowd, “What I really took out of it was the brilliance and the humility of the sailors I worked with. Not that I didn’t have that appreciation before – I certainly did – but having spent time with them to realize how their minds work and how agile and how creative they have to be. And they are constantly being tested to prove themselves to think logically, to think intuitively, and in all different matters.”

You can watch part of Butler’s visit to the Pentagon below. Summit Entertainment will release “Hunter Killer” in theaters on Friday, October 26.